![]() ![]() ramidus and the Homo- Pan LCA, (ii) resolve a long-standing debate about the role of suspension in the ancestry of humans ( 2– 4, 7, 27), and (iii) imply coevolutionary shifts in hominin hand and foot morphology associated with manipulation and obligate bipedality, respectively ( 23, 24, 28). Overall, our results (i) provide a more refined view of the positional behavior of Ar. ramidus evolved in a selective regime shared with modern chimpanzees and bonobos to the exclusion of later hominins. ![]() Second, we reconstruct the evolution of the hominin hand and show that Ar. ramidus hand and show that it is most similar to chimpanzees, bonobos, and orangutans among a sample of 53 anthropoid primate species. First, we evaluate the morphometric affinities of the Ar. Species in a selective regime share a common selective factor, e.g., positional behavior, where regime shifts imply evolution toward new adaptive optima brought on by a change in selection. We combine standard morphometric analyses with phylogenetic comparative methods that allow us to reconstruct the adaptive landscape of hominin hand evolution, placing fossil taxa within shared selective regimes. Here, we test the hypothesis that hominins evolved from an ancestor that lacked adaptations for below-branch suspension using metric data from a large and diverse sample of extant primates and fossil hominins with special emphasis on the hand of Ar. In this case, the distinctive pollical to nonpollical ray proportions of modern humans would be considered an exaptation, rather than an adaptation, useful for manipulation and tool use ( 23, 24) since it originated in the context of Miocene hominoid arboreal quadrupedal locomotion and feeding behavior ( 22, 25). ramidus did not have a suspensory-adapted hand and instead had a more generalized morphology, then human hand morphology may be less derived than that of chimpanzees and bonobos relative to the LCA ( 2, 22). ramidus was argued to be reminiscent of more “generalized” (in some ways monkey-like) Miocene hominoids such as Ekembo and Pierolapithecus. ramidus had none of the adaptations associated with forelimb-dominated suspension present among extant hominoids ( 2– 4). The initial analysis of the ARA-VP-6/500 hand concluded that Ar. The hand of the early Pliocene hominin partial skeleton ARA-VP-6/500 attributed to Ardipithecus ramidus has been argued to support the hypothesis of a nonsuspensory LCA ( 2– 4, 22). Overall, our results suggest that early hominins evolved from an ancestor with a varied positional repertoire including suspension and vertical climbing, directly affecting the viable range of hypotheses for the origin of our lineage. We identify an evolutionary shift in hand morphology between Ardipithecus and Australopithecus that renews questions about the coevolution of hominin manipulative capabilities and obligate bipedalism initially proposed by Darwin. Here, we use morphometric and phylogenetic comparative methods to show that Ardipithecus retains suspensory adapted hand morphologies shared with chimpanzees and bonobos. However, the hand of the 4.4-million-year-old hominin Ardipithecus ramidus purportedly provides evidence that the hominin hand was derived from a more generalized form. Early 20th century anatomical research supported the view that humans evolved from a suspensory ancestor bearing some resemblance to apes. The morphology and positional behavior of the last common ancestor of humans and chimpanzees are critical for understanding the evolution of bipedalism. ![]()
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